An idiom is an expression, common to a particular language, that often differs from the literal meaning of its parts taken as a whole. "A manifestation of the peculiar" is the closest possible translation of the Greek word.
In the realm of speech this may be applied to a whole language as peculiar to a people, to a dialect as peculiar to a district, to a technical vocabulary as peculiar to a profession, and so forth. For the most part, an idiom is any form of an expression that has established itself as the particular way preferred by a people-group over other forms in which the principles of abstract grammar, if there is such a thing, would have allowed the idea in question to be clothed.
Caution: Never tamper with an idiom. As Theodore Bernstein notes in The Careful Writer, you'll do so at your own peril, "and the peril is great."
Example: An editor once changed the idiom shed light to show light.He argued that light cannot be "shed." While his argument appeared to make sense, he was wrong to tamper with an idiom, for it distracted his readers and made the author appear ignorant.
Note that grammar and idiom are independent categories; being applicable to the same material, they sometimes agree and sometimes disagree about particular specimens of it. Examples of grammatical idioms:
- She was promoted captain.
- The boat all but capsized.
- Were it true...
Examples of ungrammatical (but allowable) idioms:
- Who do you take me for?
- It was not me.
- There is heaps of material.
Note: Be especially alert for prepositions that are idiomatic when following certain words and preceding others. Examples:
- "Stephanie confided her troubles to her coach."
- "Figaro confided in me."
- "Igor was impatient at the postponement."
- "Magilla was impatient with her boss."
A good book on idioms is A Dictionary of American Idioms, published by Barron's Educational Series Inc.
This tip composed with excerpts from A Dictionary of Modern English Usage by Fowler, and other sources.